1. Field of the Invention
This invention relates to a distance-measuring device for use with a camera or the like, and more particularly to a distance-measuring device based on light-projecting trigonometry, in which the distance from the device to an object is measured by projecting rays of light from the light-projecting section to the object and receiving the reflected rays of light with the light-receiving section located a specified base length away from the light-projecting section.
2. Description of the Related Art
Distance-measuring devices based on light-projecting trigonometry are well known and available with various types. For example, in the distance-measuring device disclosed in U.S. Pat. No. 4,464,038, as shown in FIG. 11A, a light projection pattern 85 is cast from a light-emitting element 80 via a projection lens 83 to an object. Then, the reflected rays of light from the object are directed by a reception lens 84 to light-receiving elements 81 and 82. Based on the outputs of the elements 81 and 82, the distance is determined. As shown in FIG. 11B, the light-emitting element 80 and light-receiving elements 81 and 82 are placed in specified positions on a single board 87. The light-emitting element 80 enclosed by an electrode section 80a is secured to one side of the board 87 and the two light-receiving elements 81 and 82 are fixed in close proximity to each other to the other side so that the boundary line between these elements 81 and 82 may be perpendicular to the direction of the base length 88.
In the distance-measuring device thus constructed, the position of the reflected-light image 89 formed on the extension of the base length 88 varies with the distance from the device to the object according to the principles of trigonometry. Because the difference in the light-receiving area between the light-receiving elements 81 and 82 varies with the position of the reflected-light image 89, the distance from the device to the object can be measured on the basis of the output difference between the light-receiving elements 81 and 82.
FIG. 12 illustrates the configuration of the optical system of the distance-measuring system shown in FIGS. 11A and 11B. Here, the light-emitting element 80 with the chip size t is located the focal length f.sub.T of the projection lens 83 away from this lens. The distance a from the device to the subject or the object 86 is determined based on the photoelectric current outputs I.sub.1 and I.sub.2 from the light-receiving elements 81 and 82 located the focal length f.sub.J of the light-receiving lens 84 away from this lens, which is the base length L away from the projection lens 83. FIGS. 13A through 13D show how the reflected-light image moves depending on the distance from device to object. When the reflected-light image 89 with an outside diameter of t.sub.J moves over the light-receiving elements 81 and 82 according to the distance from device to object, the distance calculation output I.sub.1 /(I.sub.1 +I.sub.2) based on the photoelectric current outputs I.sub.1 and I.sub.2 from the light-receiving elements 81 and 82 changes in the range of 1 to 0 depending on the distance from device to object, as shown in FIG. 13E. The range of 1 to 0 exactly corresponds to the outside diameter t.sub.J of the reflected-light image 89.
Such a conventional distance-measuring device, however, basically involves the conflicting requirements of improving the distance-measuring accuracy and expanding the distance-measuring range, which will be explained below.
In the aforementioned distance-measuring device, the reciprocal of the distance a from device to subject relates to the distance calculation output I.sub.1 /(I.sub.1 +I.sub.2) in such a manner as shown in FIG. 14A. Specifically, solid lines I.sub.1 and I.sub.2 in FIG. 14A show the relationship between the reciprocal of the distance a and the distance calculation output I.sub.1 /(I.sub.1 +I.sub.2) for light-projecting chip sizes t.sub.1 and t.sub.2 (t.sub.1 &lt;t.sub.2), respectively.
The amount of signal light reaching the light-receiving element from the subject is so small that the signal current ratio of I.sub.1 /(I.sub.1 +I.sub.2) is affected by circuit noise or the like. Variations in the output signal, or the range of noise, caused by circuit noise, as shown in FIG. 14B, have the same noise range .DELTA.l.sub.3 and .DELTA.l.sub.4 for the same distance a.sub.1 in both cases of the characteristic curves l.sub.3 and l.sub.4. The distance calculation output I.sub.1 /(I.sub.1 +I.sub.2) to which the noise component is added is indicated by a shaded portion.
To determine the distance a.sub.1 from device to subject, a reference level V.sub.a is used for the characteristic curve l.sub.3 and V.sub.b for the characteristic curve I.sub.4.
Because of the noise component, the range of uncertainty in determining the distance is .alpha. for the curve l.sub.3 and .beta. for the curve l.sub.4. That is, measuring the distance based on the curve l.sub.4 results in a wider range of uncertainty in distance determination than based on the curve .sub.3, thus lowering the distance-measuring accuracy.
The distance-measuring accuracy of the device may be evaluated based on (range of noise)/(inclination of distance calculation output). Specifically, if the distance between the projection lens and light-emitting element is f.sub.T, the base length L, and the chip size of the light-emitting element t, then the distance-measuring range S will be: EQU S=.infin.through f.sub.T .multidot.L/t
If the distance calculation output noise is N, the distance-measuring accuracy R will be: EQU R=N.multidot.f.sub.T .multidot.L/t
As seen from the these equations, the distance-measuring range S becomes greater as the focal length f.sub.T of the projection lens and the base length L become smaller. When the chip size t of the light-emitting element becomes as large as t.sub.2, as shown by the characteristic curve l.sub.2 in FIG. 14A, its expansion to f.sub.T .multidot.L/t.sub.2 on the nearest side broadens the distance-measuring range S, but the distance-measuring accuracy R gets worse. In contrast, as f.sub.T and L become larger or t gets smaller, the distance-measuring accuracy R gets better but the measuring range S becomes smaller. In other words, in such a distance-measuring device, improvements in the measuring accuracy run counter to the expansion of measuring range.
To cause improvements in the measurement accuracy to be compatible with the expansion of measuring range, it may be possible to increase the amount of light to be projected to decrease the noise range of the distance calculation output. For use with compact devices such as cameras, space for the power supply and the projection and reception sections should be as small as possible and the power supplied to the distance-measuring device be minimized. Therefore, the way to increase the amount of light to be projected cannot be accepted from viewpoints of cost and space.
The way to increase the chip size t of the light-projecting element or shorten the focal length f.sub.T of the projection lens to expand the measuring range creates a larger projection pattern than the object because the object is normally a thing of a limited size such as a person, which makes it difficult to measure the distance accurately.
To broaden the measuring range, there may be a method of tilting the boundary lines toward the direction of the base length as shown in FIGS. 15A through 15D (FIG. 15E is a finally obtained characteristic diagram). This method also has similar characteristic curves to those in FIG. 14B, meaning a deteriorated measuring accuracy.